1. Field of the Invention
The present invention generally relates to voltage regulator modules (VRMs) particularly for high-current, low-voltage applications such as the powering of microprocessors and, more particularly, to VRMs using switches which do not exhibit body diode effects and which may be self-driven.
2. Description of the Prior Art
The design of semiconductor integrated circuits and digital logic circuits, including memories, gate arrays and microprocessors in particular, has long exhibited a trend toward smaller circuit element size and increased density of circuit element integration on a chip in view of increased manufacturing efficiency and improved functionality and performance, particularly in terms of clock speed/cycle time and noise immunity, of the integrated circuit chips which can be realized thereby. However, small circuit element size and limitations on power dissipation requirements, particularly at higher clock speeds (e.g. above 1 GHz), and breakdown resistance has led to designs operating at lower voltages while the number of circuit elements integrated on a single chip has led to requirements for higher currents to power such chips. Currently, typical power supply voltages are about 1.3 volts and can be expected to decrease in future designs. It follows that the allowable difference between maximum and minimum input voltages has also greatly diminished with recent designs and may be expected to decrease further. As an example, the input voltage tolerance for a Pentium IV™ processor is only about 130 mV while corresponding current requirements currently exceed 70 A and can be expected to increase in future designs. It is projected that the next generation of microprocessors may have power requirements of in excess of 150 A at less than one volt. The current slew rate at the sensing point of the chip power supply connections may reach four to five Amperes/nsec as compared with 450 A/μsec currently required. The area which can be occupied by the power supply on the motherboard or elsewhere has become substantially fixed in designs for an extended period of time and it is also to be expected that future voltage converters/regulators and VRMs will be required to have a much increased current density compared with current circuits. The only solution to these projected requirements which is apparent at the present time is to increase switching frequency to allow smaller passive components such as capacitors and inductors to be used.
However, increased switching frequency increases switching related power losses and thus may cause problems in the thermal design of voltage regulators/converters. As switching frequency is increased, the body diode conduction loss of the so-called “bottom switch” (also referred to as a synchronous rectifier or, simply, SR) which supplies inductor current when the input switch (or “top switch”, sometimes referred to as a control switch since its conductive period controls the voltage developed on an output filter capacitor by controlling charging from the higher input voltage) is off and gate driver losses become dominant. For example, in a buck converter having a 5V input and 1.3V/12 A output, the efficiency loss attributable to body diode loss of the synchronous rectifier at a switching frequency of 1 MHz is about 2.7% whereas at switching frequencies of 2 MHz and 3 MHz, the body diode conduction losses rise to about 5% and 7.2%, respectively. Turn-off loss and gate driver loss are comparable to body diode losses in their respective effects on efficiency. Further, given the trend toward portable digital data processing devices which are often operated on battery power, such additional losses are particularly undesirable.
The body diode and gate driver losses are compounded by the fact that the top switch and bottom switch are operated in a generally complementary fashion; the bottom switch being conductive when the top switch is not and vice-versa. To avoid shorting the input power supply, a short period of “dead-time” is generally provided between the conductive periods of the top and bottom switches. Further, a MOSFET is conventionally used as the bottom switch or synchronous rectifier in a buck converter and some other voltage converter circuit types. Therefore, the body diode conducts current during the dead-time and it is known that the voltage drop across a diode, such as the body diode of a MOSFET, is much larger than the voltage drop across the MOSFET in a conductive state and the diode body power loss is thus increased over normal MOSFET conduction loss. Moreover, the MOSFET should have a very low “on” resistance since it conducts current for the predominant portion of the switching period of the voltage converter, particularly when the difference between input and output voltages of the converter is large. It follows that the gate charge on the MOSFET may also be very large which causes a large gate driver loss at high switching frequencies.